In electronic devices, such as personal digital assistants (PDAs), laptop computers, office automation equipment, medical equipment, and car navigation systems, touchscreens are widely used as their display screens that also serve as input means.
There are a variety of touchscreens that utilize different position detection technologies, such as optical, ultrasonic, surface capacitive, projected capacitive, and resistive technologies. A resistive touchscreen has a configuration in which an optically transparent conductive material and a glass plate with an optically transparent conductive layer are separated by spacers and face each other so as to function as a touchsensor formed of an optically transparent electrode. A current is applied to the optically transparent conductive material and the voltage of the glass plate with an optically transparent conductive layer is measured. In contrast, a capacitive touchscreen has a basic configuration in which an optically transparent electrode that functions as a touchsensor is formed of an optically transparent conductive material having an optically transparent conductive layer provided on a base material. Having no movable parts, a capacitive touchscreen has high durability. With such high durability and high transmission rate, capacitive touchscreens are used in various applications. Further, touchscreens utilizing projected capacitive technology allow simultaneous multipoint detection, and therefore are widely used for smartphones, tablet PCs, etc.
Generally, as an optically transparent conductive material used for optically transparent electrodes of touchscreens, those having an optically transparent conductive layer made of an ITO (indium tin oxide) film formed on a base material have been used. However, there has been a problem of low optical transparency due to high refractive index and high surface light reflectivity of ITO conductive films. Another problem is that ITO conductive films have low flexibility and thus are prone to crack when bent, resulting in increased electric resistance of the optically transparent conductive material.
A known optically transparent conductive material having an optically transparent conductive layer as an alternative to the ITO conductive film is an optically transparent conductive material having, as an optically transparent conductive layer, a mesh pattern of a metal thin line on an optically transparent base material, in which pattern, for example, the line width, pitch, pattern shape, etc. are appropriately adjusted. This technology provides an optically transparent conductive material which maintains a high light transmission rate and which has a high conductivity. Regarding the mesh pattern formed of metal thin lines (hereinafter referred to as metal mesh pattern), it is known that a repetition unit of any shape can be used. For example, in JP 10-41682 A, a triangle, such as an equilateral triangle, an isosceles triangle, and a right triangle; a quadrangle, such as a square, a rectangle, a rhombus, a parallelogram, and a trapezoid; a (regular) n-sided polygon, such as a (regular) hexagon, a (regular) octagon, a (regular) dodecagon, and a (regular) icosagon; a circle; an ellipse; and a star, and a combinational pattern of two or more thereof are disclosed.
As a method for producing the above-mentioned optically transparent conductive material, a semi-additive method for forming a metal mesh film, the method comprising making a thin catalyst layer on a base material, making a resist pattern on the catalyst layer, making a laminated metal layer in an opening of the resist by plating, and finally removing the resist layer and the base metal protected by the resist layer, is disclosed in, for example, JP 2007-284994 A and JP 2007-287953 A.
Also, in recent years, a method in which a silver halide diffusion transfer process is employed using a silver halide photosensitive material as a precursor to a conductive material is known. For example, JP 2003-77350 A, JP 2005-250169 A, and JP 2007-188655 A disclose a technology for forming a metal (silver) pattern by a reaction of a silver halide photosensitive material (a conductive material precursor) having at least a physical development nuclei layer and a silver halide emulsion layer in this order on a base material with a soluble silver halide forming agent and a reducing agent in an alkaline fluid. This method can reproduce a metal pattern of a uniform line width. In addition, due to the highest conductivity of silver among all metals, a thinner line with a higher conductivity can be achieved as compared with other methods. An additional advantage is that a layer having a metal pattern obtained by this method has a higher flexibility, i.e. a longer flexing life as compared with an ITO conductive film.
When an optically transparent conductive material having such a metal pattern as described above is placed over a liquid crystal display, the cycle of the metal pattern and the cycle of the liquid crystal display element interfere with each other, causing a problem of moire. Liquid crystal displays have elements of various dimensions depending on the screen size and the resolution, which further complicates the problem.
As a solution to this problem, in Patent Literature 1, Patent Literature 2, Patent Literature 3, and Patent Literature 4, a method in which the interference is suppressed by the use of a traditional random diagram described in, for example, Non Patent Literature 1 is suggested. In Patent Literature 5, an electrode base material for touchscreens, in which a plurality of unit pattern areas having a random metal pattern are arranged is introduced.